jtag/drivers/stlink: allow to reconnect seamlessly after polling failure

jtag/drivers/stlink: allow to reconnect seamlessly after polling failure

If the communication with the target was failing (either because of an
intermittent connection or the target was rebooted), this is needed to
reestablish operational state.

Reported-by: Tim Sander <tim@krieglstein.org>
Tested-by: Tim Sander <tim@krieglstein.org>
Change-Id: I91ea2e2b2b5ef8eb27dfe9bae95ef2a919f67e4e
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2152
Tested-by: jenkins
Reviewed-by: Tim Sander <tim@krieglstein.org>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>